Optical signals received through conventional optical links are typically distorted by significant amounts of chromatic dispersion (CD) and polarization dependent impairments such as Polarization Mode Dispersion (PMD), polarization angle changes and polarization dependent loss (PDL). Chromatic dispersion (CD) on the order of 30,000 ps/nm, and polarization rotation transients at rates of 105 Hz are commonly encountered. Various methods and systems intended to address some of these limitations are known in the art.
FIG. 1 schematically illustrates a representative coherent optical receiver capable of implementing the methods of Applicant's co-pending U.S. patent application Ser. Nos. 11/294,613 filed Dec. 6, 2005 and entitled “Polarization Compensation In A Coherent Optical Receiver”; 11/315,342 filed Dec. 23, 2005 and entitled “Clock Recovery From An Optical Signal With Dispersion Impairments”; 11/315,345 filed Dec. 23, 2005 and entitled “Clock Recovery From An Optical Signal With Polarization Impairments”; 11/366,392 filed Mar. 2, 2006 and entitled “Carrier Recovery In A Coherent Optical Receiver”; and 11/423,822 filed Jun. 13, 2006 and entitled “Signal Acquisition In A Coherent optical Receiver”, the content of all of which are hereby incorporated herein by reference.
As may be seen in FIG. 1, an inbound optical signal is received through an optical link 2, split into orthogonal received polarizations by a Polarization Beam Splitter 4, and then mixed with a Local Oscillator (LO) signal 6 by a conventional 90° optical hybrid 8. The composite optical signals 10 emerging from the optical hybrid 8 are supplied to respective photodetectors 12, which generate corresponding analog electrical signals 14. The photodetector signals 14 are sampled by respective Analog-to-Digital (A/D) converters 16 to yield raw multi-bit digital signals 18 corresponding to In-phase (I) and Quadrature (Q) components of each of the received polarizations.
The resolution of the A/D converters 16 is a balance between performance and cost. It has been found that a resolution of n=5 or 6 bits provides satisfactory performance, at an acceptable cost. The sample rate of the A/D converters 16 is selected to satisfy the Nyquist criterion for the highest anticipated symbol rate of the received optical signal.
From the A/D converter 16 block, the respective n-bit I and Q signals 18 of each received polarization are supplied to a respective dispersion compensator 20, which operates on the raw digital signal(s) 18 to at least partially compensate chromatic dispersion of the received optical signal.
The dispersion-compensated digital signals 22 appearing at the output of the dispersion compensators 20 are then supplied to a polarization compensator 24 which operates to compensate polarization effects, and thereby de-convolve transmitted symbols from the complex signals 22 output from the dispersion compensators 20. If desired, the polarization compensator 24 may operate as described in Applicant's co-pending U.S. patent application Ser. No. 11/294,613 filed Dec. 6, 2005. The output of the polarization compensator 24 is a pair of multi-bit estimates 26 X′(n) and Y′(n) of the symbols encoded on each transmitted polarization. These symbol estimates 26 X′(n), Y′(n) contain both amplitude and phase information of each transmitted symbol, and include phase error due to the frequency offset between the Tx and LO frequencies, laser line width and phase noise. In some embodiments, the symbol estimates 26 are 10-bit digital values, comprising 5-bits for each of real and imaginary components of each symbol estimate.
The symbol estimates 26 X′(n), Y′(n), appearing at the output of the polarization compensator 24 are then supplied to a carrier recovery block 28 for LO frequency control, symbol detection and data recovery, such as described in Applicant's co-pending U.S. patent application Ser. No. 11/366,392 filed Mar. 2, 2006.
As may be seen in FIG. 2, the carrier recovery block 28 is divided into two substantially identical processing paths 30; one for each transmitted polarization. Each processing path 30 receives a respective stream of symbol estimates 26 output by the polarization compensator 24, and outputs recovered symbols 32 of its respective transmitted polarization. Each processing path 30 includes a decision circuit 33 and a carrier recovery loop comprising a carrier phase detector 34 and a phase rotator 36. In general, the phase rotators 36 use a carrier phase estimate generated by the respective carrier phase detector 34 to compute and apply a phase rotation K(n) to the symbol estimates X′(n) and Y′(n) received from the polarization compensator 24. The resulting phase-rotated symbol estimates X′(n)e−jk(n) and Y′(n)e−jk(n) are assumed to lie within the correct quadrant of the QPSK phase diagram. Accordingly, the decision circuits 40 use each successive phase-rotated symbol estimate X′(n)e−jk(n) and Y′(n)e−jk(n) to generate corresponding recovered symbol values X(n) and Y(n) having ideal phase and amplitude values for the quadrant in which the phase-rotated symbol estimates are located. The phase detectors 34 operate to detect respective phase errors Δφ between the rotated symbol estimates X′ (n)e−jk(n) and Y′ (n)e−jk(n) and the corresponding recovered symbol values X(n) and Y(n). This operation is described in detail in Applicant's co-pending U.S. patent application Ser. No. 11/366,392 filed Mar. 2, 2006.
An advantage of the system of FIGS. 1 and 2 is that reliable signal acquisition, clock recovery, compensation of dispersion and polarization effects, carrier recovery and Local Oscillator (LO) control can be accomplished, even in the presence of moderate-to-severe optical impairments. Taken in combination, these elements enable the deployment of a coherent optical receiver in real-world optical networks, with attractive signal reach and line rate characteristics. A limitation of this system, however, is that in severely impaired environments, the bit error rate (BER) of the recovered symbol values X(n) and Y(n) may be undesirably increased by residual Inter Symbol Interference (ISI) and DC offset in the rotated symbol estimates X′(n) e−jk(n) and Y′ (n)e−jk(n).
Accordingly, methods and techniques that enable cost effective compensation of DC offset and residual ISI in a receiver unit of an optical network remain highly desirable.